Intensification of an electron beam from cold-cathode discharge



Jan. 25, 1955 J. H. PARK 2,700,743

INTENSIFICATION OF AN ELECTRON BEAM FROM COLD-CATHODE DISCHARGE FiledOct. 5, l95l 2 Sheets-Sheet 1 PULSE 1 H VOLTAGE R /E H 8 TIME IN #556.

INVENTOR.

gumwwe Jan. 25, 1955 J. H. PARK 2,700,743

INTENSIFICATION OF AN ELECTRON BEAM FROM COLD-CATHODEJ DISCHARGE FiledOct. 5, 1951 2 Sheets-Sheet 2 C8 {+5Kv 1\NV\ I! 5 2/ 70 CRO JVORINDERRELAY PLATES IN VE NTOR.

INTENSIFICATION F AN ELECTRON BEAM FROM COLD-CATHODE DISCHARGE John H.Park, Bethesda, -Md., :assignor Ito the United States of America asrepresented by the Secretary f Commerce Application October 5, 1951,Serial No. 250,004

6 Claims. (01. BIS-+3.0)

(Granted under Title 35, U. S. Code('1952), sec. 266') The inventiondescribed herein may be manufactured and used by or for the Government,of the United States for governmental purposes without the payment tome of any royalty thereon in accordance with the provisions of the actof March 3, 1883, as amended (45 Stat. 467; U. S. C.

The present invention is concerned with the momentary intensification ofan electron beam obtained from a "highvoltage cold-cathode dischargetube and more specifically is concerned with intensifying the electronbeam for afew microseconds so that very rapid electrical transients maybe photographed.

High-voltage cold-cathode discharge tubes have .long been used as asource of electron beams. Probably their most important application hasbeen to supply the-electron beam for high-speed cathode-rayoscillograp'hs which :are used in the recording of transient electricalphenomena of short duration. in the past it has been very difficult toobtain photographic records of some of the shorter transients. The speedat which an electron beam must be deflected to follow the transients ofsuch short duration is very great and in the past it has notbeenposs'ib'le too'btain a beam of suffic'ient intensity to affectphotographic papers while being deflected at such high speeds.

A number of attempts have been made in the past to obtain a beam ofsufficient intensity by shaping of the electrodes, using different gasesin the discharge tube and using prefocusing coils placed around thedischarge tube. However, to date the normal maximum writing speed forsuch oscillographs has been about 200 inches 51361 microsecond.

The primary object of this invention is to provide a cathode-rayoscillograph with an electron beam of sufficient intensity to allowoscillographs to be made of very short duration electrical transients.

Another object of the invention is to provide a cathoderay oscillographwith an electron beam with a very high writing speed.

Still another object is to provide an electron beam capable of affectingphotographic papers at a writing speed of approximately three-fourthsthe speed of light.

Still another object of the invention is to provide for a momentaryintensification, of '50 to times, of an electron beam emitted by ahigh-voltage, cold-cathode discharge tube.

Another object is to provide -a large momentary intensification of theelectron beam while suppying only a comparatively small increase inanode potential.

Another object of this invention is to provide an electron beam ofsufiicient intensity to record on photographic papers electricaltransients lasting two microseconds or less.

Another object is to provide a high-voltage cold-cathode discharge tubehaving an electron beam of sufficient intensity to allow photographicrecords to be made at writing speeds of approximately 9,100 inches permicrosecond.

in accordance with the present invention the electron beam discharge ina cold-cathode discharge tube is greatly intensified for a short periodby superimposing a voltage pulse of about 2 percent of thecathode-to-anode voltage on either the anode or the cathode.

Other uses and advantages of the invention will become upon reference tothe specification and drawings.

Figure 1 is a circuit diagram of one form of the invention.

Figure 2 is a copy of an oscillogram showing the time lags involved inthe present invention.

part'of the present invention.

2,700,743 Patented Jan. 25, 1955 Figure 3 is a circuit diagram ofanother form of the invention.

Figure 4 is a circuit diagram of the modified cathoderay oscillographvoltage supply.

Referring now to hgure i, the discharge section of the high-voltagecold-cathode discharge tube which may be of the type shown in UnitedStates Patent No. 1,919,560 is shown at 1. The cathode 2 is connected bywire 3 through the resistors R1 and R2 to the negative highvoltagesupply 15. The junction between R1 and R2 is grounded through thecondenser C1. The discharge section is enclosed in a glass envelope 4which is sealed into the cathode by rubber gasket 5 and is sealed to theanode :6 at "7. Before modification for purposes of the presentinvention the anode 6 was fastened directly to the all metal outercasing '8 of the cathode-ray oscillograph chamber 9 and was thereforegrounded. I However, after modification 'for reasons to be explainedlater, the anode 6 is electrically insulated from the outer casing "8 bythe insulator '11. The anode 6 is then connected to groundthroughresistorRS. v x

The operation of the device is as "follows: With '40 to 60 kilovoltsdirect current applied between the cathode 2 and the anode 6 and thepressure adjusted to approximately 5 to 10 "microns of mercury, a fine{pencil line discharge is obtained originating in'a bright spot at "thecenter of the cathode. By regulating the conditions in the dischargeregion, the discharge current can be made to have a value of from 0:1'to 0.8 milliampere. About one percent of the cathode current passesthrough the small hole 12 in the center of the'anode "6 into t'h'ema'inchamber 9 forming the electron beam 10. Here the beam passes through theusual beam trap, focusing coils, deflecting and sweep plates, andfinally strikes the film. None of these are shown in Figure l and theyform no The transient to be studied is applied to one set of thedeflection plates and causes a deflection of the electron beam, a sweepvoltage being applied to the other set of deflection plates, thusleaving a trace on the film which is in eifect a plot of voltage againsttime for the transient. A's 'pi'eviouslyjpointed"out, it has 'been veryditlicult in the past to obtain an electron beam of suificient intensityto leave a trace on the film when .moving at very :high speeds. Toovercome this in accordance with 'the present invention 'a very briefpositive current pulse is passed through resistorfR'S to ground. Thecurrent pulse is only large enough to increase the voltage from cathodeto anode by from 800 to 2500 volts over the existing 50 "kilovolts,However, this causes a beam intensification of up to '50 times normalintensity. The increased beam intensification lasts "for a very shorttime (about 2 microseconds) and therefore does not appreciably increasethe burning of the cathode.

When the voltage pulse is applied to the anode "the discharge currentincreases to a high value immediately. Beam intensification and anotherincrease in discharge current occur a short time later.

This can be seen clearly with reference to Figure 2. The zero currentline 13A was made with a beam of "nor mal intensity. At zero time an1800-volt positive pulse was applied to the anode 6 in a manner to bedescribed later. The current, as shown by trace 13, increasedimmediately to 7 milliamperes as shown at A, but the electron beam wasnot intensified. However, with-inle's's than 0.5 microsecond the currentagain increased (-11 111a,), as shown at B, but this time the electronbeam was greatly intensified. This is shown by the fact that the tr-acei3 is very thick and bright at this point. The voltage pulse was only1800 volts and therefore this trace is used to show typical rather thanoptimum results.

The delay between the application of the voltage pulse and the desiredbeam intensification is dependent upon two factors. One is the magnitudeof the voltage pulse and the other is the initial electron beam current.At present, delays of only 0.25 microsesecond have been obtained. Thisvery short delay can be obtained if a pulse of 2500 volts is usedregardless of how low the initial value of the beam current is. On theother hand if fa voltage pulse of 800 volts is used, a delay of only0.25 microsecond cannot be obtained no matter 'how large the initialbeam current.

Figure 3 shows a circuit used to apply a negative voltage pulse to thecathode. This type of operation also increases the cathode-to-anodevoltage and the results obtained are identical with those obtained whenthe anode is pulsed.

In this modification the anode is grounded and therefore the specialinsulator 11 used in the modification of Figure 1 can be eliminated.However, here the resistor R1 and capacitor C1 are shunted by thecapacitor C2 and resistor R4.

With the steady direct-current voltage on the cathode the capacitor C2is charged to 50 kilovolts. By passing a current through the resistor R4from ground to point 14, the point 14 becomes negative with respect toground. '1'h1s decreases the cathode voltage and increases thecathode-to-anode voltage.

It will be noted that beam intensifications of up to 50 times areobtainable with sudden increase in cathode-toanode voltage of only 2percent. As a possible explanation of this phenomenon consider thevariation of net space charge density in the discharge between thecathode and anode. There is a high concentration of both electrons andpositive ions throughout the volume of the discharge. Electrons arebeing constantly fed into the region near the cathode, and since ittakes a definite, even though short, time for them to be acceleratedtoward the anode by the impressed electric field, a large negative spacecharge region is built up near the cathode. In the region near theanode, including the space inside the tube forming part of the anode,gas molecules are continually being ionized by the short wave radiationproduced by the impact of high speed electrons on the anode. The slowelectrons produced in this manner drift toward the anode and leave anexcess of positive ions which tend to be accelerated toward the cathodebut in the meanwhile build up a high net positive spacecharge regionnear the anode.

Under steady-state conditions with 50 kilovolts direct current appliedbetween the cathode and anode and with normal electron beam, it may beassumed that the high concentration of negative space charge in theregion near the cathode makes the gradient at the cathode quite small incomparison to the average gradient between electrodes. Similarly thegradient near the anode should be quite small though of opposite sign.It must be kept in mind with respect to both of these space chargeregions that although they have a definite sign they both containparticles of the opposite sign in each region. That is, the negaivespace charge around the cathode contains many positive ions and thepositive space charge contains many electrons. When the voltage betweencathode and anode is suddenly increased, an instantaneous change incharge can occur only on the electrode surfaces. Since the electrodemore nearly approximate points rather than infinite planes, the suddenchange in their surface charge causes a sudden increase in gradientmainly in the regions near the electrode. The sudden increase ingradient causes electrons from the positive space charge and respectivepositive ions from the negative space charge to be drawn from each spacecharge region toward the respective adjacent electrodes. This flow ofelectrons and ions constitutes a current flow in the external supplycircuit which is superposed on the steady state current in the dischargetube. Electrons and ions flow in a direction to increase the negativespace charge density in the cathode region and to increase the positivespace-charge density in the anode region. This is illustrated by thefirst part of the trace 13 in Figure 2. A sudden increase of availableelectrons in the space near the cathode, if it is of sufficientmagnitude, will disrupt the equilibrium conditions existing in thedischarge as shown in Figure 2 by the second increase in dischargecurrent accompanied by beam intensification. A possible explanation ofthis sudden change in equilibrium conditions in the discharge is asfollows: The increase of negative space charge in the region near thecathode causes an increase (probably gradual at first) in magnitude ofthe electron stream from cathode space-charge region to anodespace-charge region. The greater the number of fast electrons reachingthe anode the greater will be the supply of positive ions; and they willbe accelerated toward the cathode and produce more electrons. Thus theaction is cumulative. If the initial change is small or occurs at a lowrate, a new equilibrium condition will be attained with little 'eflzecton the electron beam.

If the initial disturbance is of suflicient magnitude and occurssuddenly, the cumulative efiect causes beam intensity and dischargecurrent to increase rapidly (see Figure 2). This effect is stopped verysoon because the number of positive ions that can be produced in thisshort time is limited by the low gas pressure (total number of gaseousmolecules) in the tube. As the number of available positive ions isreduced, the beam intensity and discharge current gradually decrease andin some cases the discharge is momentarily extinguished.

Figure 4 is a wiring diagram of the circuits used to supply the currentpulses necessary to produce the required voltage pulse that is to beapplied to the anode or cathode of the tube. The cathode-rayoscillograph employed in the beam intensification studies uses a pulsesource to supply the Norinder relay or beam unblocking plates. Sincethis supply is always available it is convenient to use it for the pulsesupply to the discharge electrode although it is not necessary to dothis. Another source of voltage could be used.

When the -3 kilovolt and +3 kilovolt sources are energized thecapacitors C4, C5, C6, and C7 become charged. The condenser C8 ischarged through resistors R5 and R8 through R11. The grid 17 of gas tube18 is held at the same potential as the negative plate of capacitor C8which is --3 kilovolts after C8 becomes fully charged. When the switch16 is closed, capacitor C8 discharges through resistors R6 to R11,causing the junction between R10 and R11 to go positive. This positivepulse is used to trigger the phenomenon to be studied. At the same timethe potential of the junction between resistors R6 and R7 becomes lessnegative which after a short time delay produced by C9 fires the tube18. This closes the circuit including tube 18, capacitors C5 and C7 andresistors R12, R13, R14, and R15. The capacitors C5 and C7 aredischarged and produce a large voltage pulse across resistors R12, R13,R14, and R15. As indicated in Figure 4, this voltage supplies theNorinder relay plates. This pulse is also i used to supply the 800 voltsto 2500 volts for the cathode or anode of the discharge tube and istaken off at either point 19 or point 21. The pulse taken from point 19is positive and therefore would be connected to the resistor R3 ofFigure 1. The pulse taken from point 21 is negative and therefore wouldbe applied across resistor R4 of Figure 3.

When the tube 18 fired, the capacitors C4 and C6 were also discharged.They discharged through a path consisting of tube 18, resistors R16 andR17, delay lines 22 and 23, and ground.

This circuit gives positive synchronization of the beam intensificationwith the sweep and the Norinder relay voltage, but since the time toattain maximum intensification is about 0.25 microsecond, delay lines of0.25 microsecond must be inserted in the supply to the sweep plates 24.This delay in sweep supply has the added advantage of allowing thevoltage on the Norinder relay plates to reach a high uniformly changingvalue before the sweep is started, which is quite important for veryshort sweep times.

It will be noted that the only modification necessary in the voltagesupply circuit was the addition of the delay lines. The only othermodifications necessary were the addition of an insulating ring and aresistor in Figure l or the addition of a resistor and capacitor inFigure 3. In accordance with the present invention it has been possibleto obtain beam intensifications up to 50 times normal intensities bymeans of the minor modifications pointed out above.

It will be apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementwithin the scope of my invention as defined in the appended claims.

I claim:

1. A circuit for intensifying the electron beam of a coldcathodedischarge tube comprising a cold cathode, an anode with a small hole inthe center, means for applying a direct-current voltage of from 40 to 60kilovolts between said anode and said cathode, means for intensifyingsaid electron beam including a source of pulse voltage of from 800 to2500 volts and means for applying said pulse voltage so as to increasethe anode to cathode voltage.

2. A circuit for intensifying the electron beam of a cold-cathodedischarge tube comprising a cold cathode, an anode with a small hole inthe center, means for applying a direct-current voltage of from 40 to 60kilovolts between said anode and said cathode, means for intensifyingsaid electron beam including a source of pulse voltage of from 800 to2500 volts and means comprising resistor means for applying said pulsevoltage so as to increase the anode-to-cathode voltage.

3. A circuit for intensifying the electron beam of a. cold-cathodedischarge tube comprising a cold cathode, an anode with a small hole inthe center, a resistor connected from said anode to a point of zeropotential, a negative direct-current voltage source of from 40 to 60kilovolts connected between said point of zero potential and said coldcathode and means for intensifying said electron beam including a sourceof positive pulse voltage of from 800 to 2500 volts connected acrosssaid resistor.

4. A circuit for intensifying the electron beam of a cold-cathodedischarge tube and for recording very short duration electricaltransients comprising a cold cathode, an anode with a small hole in thecenter, a resistor connected from said anode to a point of zeropotential a negative direct-current voltage source of from 40 to 60kilovolts connected between said point of zero potential and said coldcathode, means for intensifying said electron beam including a source ofpositive pulse voltage of from 800 to 2500 volts connected across saidresistor and means for applying a pulse across said resistor in timedrelation with the electrical transient.

5. A circuit for intensifying the electron beam of a cold-cathodedischarge tube comprising a cold cathode, an anode with a hole in thecenter, said anode being connected to a point of zero potential, a largenegative voltage source connected between said cathode and the point ofzero potential, a resistor and a condenser connected in series, saidcondenser also being connected to said cathode and said resistor alsobeing connected to said point of zero potential and a source of negativepulse voltage connected across said resistor.

6. A circuit for intensifying the electron beam of a cold-cathodedischarge tube to record very short duration electrical transients,comprising a cold-cathode discharge tube having a cold cathode and ananode with a small hole in the center, means for applying a constantdirect-current voltage of from to kilovolts between said anode and saidcathode to produce an electron beam of substantially constant value,means for intensifying the steady-state electron beam by a factor ofapproximately fifty, said means including a source of pulse voltagehaving an amplitude equal to about 2 to 4 percent of the initialanode-to-cathode voltage, and means for applying said pulse voltage soas to increase the anodeto-cathode voltage in timed relation with theelectrical transient to be recorded.

References Cited in the file of this patent UNITED STATES PATENTS1,977,398 Morrison Oct. 16, 1934 1,997,356 Bryant Apr. 9, 1935 2,050,411Barthelemy Aug. 11, 1936 2,152,487 Knoll Mar. 28, 1939 2,161,316Rogowski June 6, 1939 2,363,359 Ramo Nov. 21, 1944 2,368,328 RosencransJan. 30, 1945 2,408,039 Busignies Sept. 24, 1946 2,611,884 Webster etal. Sept. 23, 1952

